Electrochemical detector and voltammetric cells are known in the art and have been used with success for the analysis of flowing solution in the laboratory. Two-electrode and three-electrode cells are known. The three-electrode cell comprises a working electrode, a counter-electrode and a reference electrode which has the function of establishing and maintaining a constant potential relative to the working electrode or the sample solution. The sample solution is flown continuously through the cell. In principle, the electrodes may be affected by poisoning due to absorption with resulting passivation and loss of signaL In order to avoid such poisoning, the dropping mercury electrode has been adopted in many such cells.
U.S. Pat. No. 3,922,205 describes the basic structure of a polarographic-cell. U.S. Pat. No. 4,138,322 discloses a structure of shielded dropping mercury cathode. U.S. Pat. No. 4,260,467 describes a dropping mercury electrode which comprises a reservoir for liquid mercury, a mercury capillary at the outlet end of which mercury drops are formed, and a valve for selective air-purging passage of mercury from the reservoir to the inlet end of the capillary. An automated polarographic cell is described by C. N. Yarnitzky in Analytical Chemistry, Vol. 57, No. 9, August 1985, p. 2011-2015.
The efficiency of polarographic cells of the aforesaid type depends on the combination of a number of structural and fuctional features. A fully satisfactory combination, providing an industrially efficient such cell has not been achieved so far in the art. The cells which are automatic and also on-line are expensive and not adequately efficient. In many cases, the prior art cells use a solid electrode which becomes polluted with time, so that the cell ceases to be reliable. In on-line, in-flow cells, the signal obtained is often proportional to the Reynolds number. Because of this, attempts have been made to design small cells, having high Reynolds number, comprising means for producing and controlling the dropping of the mercury electrode. Such means, however, being complicated and unreliable. Other cells are objectionable in that they require a very large volume of the sample solution, with resulting waste of time and chemicals.
High sensitivity and short reaction time, viz. quick response, are particularly important in on-line cells and esing cells are not satisfactory in this respect. If, for instance, metal pollution occurs in an on-line system in which such a cell is inserted, a delayed reaction on the part of the cell and the consequent failure to reveal the pollution until a significant period of time has elapsed and the pollution may have reached a high level, negatively affects the operation of the system.
Among the specific problems encountered by polarographic cells employing a dropping mercury electrode of the prior art, two are particularly important. Firstly, the presence of oxygen in sample solution having a strong negative effect on the accuracy of the measurements, the oxygen must be removed as fully as possible before all the solution is fed into the polarographic cell proper. For this purpose, it has been proposed to cause the sample solution to flow in a thin layer, together with a stream of nitrogen, in a tube which leads it close to the position at which the mercury drops are formed. This arrangement, however, has been found to be unsatisfactory, and it is believed that this is due to the fact that it does not assure that the space between the counter-electrode, the reference electrode and the working electrode be always full with the sample solution, and consequently, the electrical contact between the electrodes is interrupted at times, thereby requiring interrupting the measurement and starting it anew.
Another important problem that the prior art has not satisfactorily solved, is to synchronize the measurements with the formation and the fall of the mercury drop. For the measurements to be reliable, they must be made when the drop that constitutes the working electrode has a given area. Since the area of the drop increases during its formation and until the drop is detached from the capillary from which the mercury issues, the need arises to control the separation of the drop and synchronize it with the measurements. For this purpose, it has been proposed to employ mechanical means, such as a hammer device which imparts to the mercury capillary a series of blows, to cause the detachment and fall of the mercury drop at a desired time. This system, however, is unreliable and further subjects the capillary to mechanical stresses and solution creeps in, which considerably shorten its life.
It is a purpose of this invention to provide an electroanalytical voltammetric cell, particularly, but exclusively of the dropping mercury electrode (DME) type, which is free from the drawbacks of the prior art cells.
It is another object of the invention to provide such a cell in which the electrical contact, via the sample solution, between the counter-electrode and the working electrode is constantly assured.
It is a further purpose of this invention to provide such a cell which has a very short reaction time and a very quick response.
It is a still further purpose of this invention to provide such a cell having a dropping mercury electrode, in which the measurement of the current passing through the same (which has a short, but significant duration, typically of a few seconds) is synchronized with the fall of the mercury drop, and its beginning and its end occur at predetermined, constant times of said drop, without applying mechanical stresses, such as blows, to the apparatus element where the mercury drop is formed.
It is a still further purpose of the invention to provide all of the aforesaid features and advantages with a structure that is simple, reliable, inexpensive to make and durable in operation.
It is a still further purpose of the invention to provide an electrochemical voltammetric cell having an optimal combination of structural and functional features, to permit efficient and reliable industrial use.
Other purposes and advantages of the invention will appear as the description proceeds.